OmniGrowl 4.3.3, released today, adds further improvements to iTunes artwork handling.
First, the feature to add artwork found on the internet to the iTunes album has been fixed. It has been broken since iTunes 10.2.
Second, OmniGrowl will now show separate artwork for each track of an album under specific conditions. Normally, OmniGrowl only checks artwork once per album in order to reduce the drain on iTunes.
OmniGrowl has updated Growl to 1.2.2 under the hood, and now includes the Growl installer, so it can either update Growl or install it when not present.
As usual, please see the version history for details.